1. Technical Field
The invention relates to drives for power transistors, wherein the drive is used for controlling a power transistor during a turn-on, turn-off and current-limiting operation.
2. Description of Related Art
Self-commutated converters are presently used in a number of applications, for example in static converters for motor drives, supply devices, UPS systems, etc. In such applications, an individual power transistor is normally used for taking up the relatively high voltages occurring.
There are a number of types of power transistors, the most common being MOSFET and IGBT transistors, bipolar transistors and Darlington transistors. The principles of drives disclosed through this description can be applied to all types of power transistors, but this description is substantially directed to drives in connection with voltage-controlled power transistors such as IGBTs. In the following, when referring to the electrodes of a power transistor, the designations gate, emitter and collector are used, wherein the term emitter encompasses the word "source" used in MOSFET literature and, correspondingly, the term collector encompasses the word "drain".
The following description describes a few different reasons for the desire to control the turn-on and turn-off operations in power transistors. In this connection, the concept dv/dt control and the concept di/dt control will be used. These concepts refer to methods which make it possible to control or limit the voltage derivative and the current derivative, respectively, in connection with turn-on and turn-off of the transistor. The concepts turn-on and turn-off are used as designations for switching the power transistor on and off, respectively.
During turn-on of a power transistor, for example an IGBT, the turn-on process is controlled by controlling the gate of the IGBT to prevent overload of a reverse-voltage diode (opposite diode) associated with the IGBT and caused by too fast current growth (di/dt) or too fast voltage breakdown (dv/dt) across the power transistor. This is of particularly great importance for diodes of a higher voltage (.gtoreq.1600 V). High-voltage diodes normally have a relatively large recovery charge due to a relatively high charge-carrier lifetime. This, in combination with a low doping level in the high-field region, makes the diode more sensitive to dynamic avalanche or avalanche injection during turn-on, which may be harmful to the diode. This leads to a need to limit the amount of a current derivative (di/dt), which is negative during the turn-on operation, and the voltage growth (dv/dt) to values which are acceptable to the diode. This can be done by turning on the opposite transistor smoothly, that is, by maintaining the voltage derivative at the gate low. At the same time, it is desired to keep the turn-on losses for the power transistor and the turn-off losses for the diode as small as possible.
During turn-off of the power transistor, control via the gate is used to check the voltage growth (dv/dt) for several different reasons. By means of dv/dt control it is possible to limit the amplitude of that voltage overshoot which always occurs upon turn-off of a current in an inductive circuit. This may be necessary to limit the stresses on the transistor in accordance with what is stated in the data sheets of the transistor. Control of the voltage derivative is also normally necessary upon turn-off of the power transistor when a short circuit or so-called arc-through occurs (in connection with current limiting of the power transistor). Without control or limitation of the voltage derivative (dv/dt), the transistor device can easily be damaged during turn-off of a short-circuit pulse because of the fact that high peak voltages otherwise easily arise.
When utilizing IGBTs of higher voltages (e.g. IGBTs in the voltage range.gtoreq.1600 V), there are, in addition, other difficulties. The SOA (Safe Operating Area) of the IGBT becomes dependent on the voltage derivative at which turn-off occurs. By limiting the voltage derivative, higher currents can be turned off or, alternatively, higher peak voltages can be tolerated. This can be explained by the fact that dv/dt control, which is correctly performed, results in the electron injection continuing for a considerable part of the turn-off operation, which suppresses the process which creates a dynamic avalanche such that higher currents/voltages can be tolerated during the turnoff operation than what would otherwise be the case.
Dv/dt control can also be used to limit dv/dt to which a load (e.g. a motor) is subjected, for a high dv/dt may entail local stresses on an insulation in the load, which may successively break down the insulation. Likewise, high voltage derivatives, dv/dt, may give rise to voltage transients which are transmitted out on a cable, are reflected and may generate voltage spikes which may result in insulation problems. High dv/dt values may also give rise to radio interference or disturb other electronic equipment. To fulfil the EMC standards (EMC=Electrom Magnetic Compatibility), it may be necessary to design filters which reduce these disturbances. Dv/dt control may then be an aid in attacking the problem, so to speak, at the source.
A negative factor to be considered is that control of the voltage derivative dv/dt normally increases the turn-on and turn-off losses to a certain extent. With full control of the turn-on and turn-off operations, however, the losses may be minimized. There is always an optimum way which the transistor can be turned on and turned off, respectively, if the aim is to minimize the turn-on and turn-off losses, respectively. One condition is that this optimum way is known and that the control parameters can be adapted such that the transistor is always turned on/turned off in this optimum way.
There are also other reasons why it is desirable to control the turn-on and turn-off operations of a power transistor. One such case where particularly high demands are made on the control is during series connection of transistors. During such series connection of power transistors, where individual transistors are intended to take up part of a high voltage by voltage division, there are a number of factors to consider. Some of the most important problems which have to be solved are:
static voltage division; PA1 dynamic voltage division; and PA1 voltage division under short-circuit conditions.
Of these above-mentioned problems, the primary task of this description is to seek a solution to the question of how dynamic voltage division is achieved in an optimum way, that is, during turn-on and turn-off of the transistor. Various proposed methods for this are known. Among other things, there are several different methods whereby external voltage-dividing elements are used, for example a combination of a diode, a resistor, and a capacitor. These methods, however, do not result in a solution to the problem at the source but only result in an attempt to limit the differences in voltages which arise across individual transistor modules in a chain of series-connected transistors to a level which may be tolerated, by adding external components. This increases both the volume and the cost of, for example, a device in the form of a converter designed in accordance with such known principles.
In currently used converters, a very simple method of controlling or limiting dv/dt and di/dt, respectively, is normally used. A common way is that, during turn-on of the transistor, the gate is connected to a voltage source by means of a resistor, a so-called gate resistor. This resistor will limit the current delivered by the voltage source, whereby, by a suitable choice of resistor, it is possible to influence how fast the turn-on occurs. In the same way, a combination of a voltage source and another resistor may be used to influence how fast the turn-off is to occur. This method is simple and frequently used, but it only provides limited control possibilities. During turn-on of the transistor, it is not possible to influence di/dt and dv/dt, respectively, separately. Likewise, for example, the voltage derivative during turn-off is greatly dependent on at which current the turn-off occurs (the higher the current, the greater the voltage derivative). To avoid too high a voltage overshoot during turn-off of short-circuit currents, a method is often used in which it is first detected whether the on-state voltage of the transistor exceeds a given level. If this is the case, the current is judged to be so high that turn-off must be performed smoothly, that is, with a reduced voltage derivative, which means that turn-off is carried out with a higher gate resistance than what is normally the case.
The turn-off and turn-on operations at a given current are also dependent on the temperature of the transistor. Further, large variations in the turn-off and turn-on operations may occur when different specimens of transistors of the same type are tested with the same drive. Such variations may give problems with transient current division in case of parallel connection of transistors and with transient voltage division in case of series connection of transistors. Thus, there is a need of a method which in a better way can make possible control of the turn-on and turn-off operations, respectively, of power transistors, partly with the aim of controlling the process better in its details to achieve a more optimum turn-on and turn-off, partly with the aim of reducing the dependence of the turn-on and turn-off operations on current, temperature and naturally occurring variations of transistors of the same type.
Because the known solutions are not the optimum solutions to meet various requirements, the use of an "intelligent" drive, shown according to the invention, for control of a power transistor, such as an IGBT, in its so-called linear or controllable region during switching is proposed. The solution of the present invention is somewhat more complicated than the method which is most commonly used today involving adaptation of the so-called gate resistors, but the costs incurred therefor in many applications may be more than worthwhile since the power transistors may be utilized more efficiently due to a more optimum control.